The present disclosure relates to corrosion protection of metallic conductors, and at least some embodiments relate to corrosion protection of buried or embedded metallic conductors through the use of conductive coatings.
Corrosion protection and/or coatings used for various purposes are described in a variety of literature, including for example the following U.S. Pat. Nos. 3,334,040; 4,908,157; 5,098,771; 5,476,612; 5,700,398; 5,976,419; 7,405,247; 7,422,789; 7,578,910; 7,745,528; and 7,794,626.
Over the years the economic cost of the rusting of iron-containing articles, and the electrolytic corrosion of copper conductive wiring has prompted considerable effort to find effective and economical ways to prevent such degradation. It is well known that the electrolytic corrosion of metals is a chemical process during which the metal becomes the anode in an electrical cell involving micro cells which commonly arise as a result of contact between the metallic atoms and contaminants which have different positions on the galvanic scale being more cathodic. Since water is essential for galvanic corrosion to proceed, and since oxygen frequently accelerates the process, most methods commonly employed to mitigate such corrosion involve isolating the metals from air and water. For this purpose barrier coatings including paints, impermeable polymers as well as certain types of metal insoluble metallic salts are frequently employed.
An alternative approach used to protect metals from corrosion involves the use of so-called sacrificial coatings which takes advantage of the Galvanic Series. In this methodology the susceptible metal is mechanically connected to another with a lower electronegative potential. When subjected to conditions favoring electrolysis these more reactive metals are consumed in preference to the more cathodic structure which are protected. Common sacrificial anodes include such metals as aluminum, magnesium, tin or zinc and their alloys.
While the use of sacrificial anodes is not of direct interest to this disclosure, the utilization of non-galvanic electrical conductors, and various polymeric binders to improve the cost effectiveness of the sacrificial anodes is relevant.
A widely used alternate approach for corrosion protection of steel, of particular relevance to buried pipelines, involves connecting the metallic structure to a source of direct electrical current in such a way that the metal to be protected becomes the cathode of the electrolytic cell, and thus preventing corrosion of the protected metal. Although the anode in such a cathodic protection (CP) circuit is not strictly ‘sacrificial’, in reality a certain amount of erosion of the anode does occur over time as the result of electrochemical activity at the interface of the anode and the surrounding conductive material, which is commonly moist soil.
Most commonly the rate of erosion of such anodes is managed by choice of oxidation resistant conductive materials such as mixed metal oxides or ferrosilicon. Various additional techniques have been developed in order to further extend the lives of these expensive materials, one common method being utilization of one of a number of types of carbonaceous backfill, which over time becomes preferably oxidised by a non-galvanic oxidative mechanism, thus protecting the more valuable metallic anode. An alternative approach to the use of carbonaceous backfill involves various types of conductive carbonaceous compositions which are installed in direct contact with the metallic anode. It is necessary that such protective materials have the ability to both protect the steel surface from air and moisture, and also to allow the egress of such gases which might be generated in the oxidative environment of the anode.
While many of the above processes can be used to protect anodes subject to deliberately induced electrical currents, there is also a requirement to protect steel structures and buried copper cable utilized for grounding purposes, which are subject both to accidentally induced currents which render such installations anionic, and chemically corrosive underground water systems. Existing systems do not provide an acceptable solution to this particular problem in which buried metals that for one reason or another are affected by stray direct electrical current in such a manner that they become anodes in a galvanic cell. The economic cost of such corrosion is extremely high.